1. Field of the Invention
This invention relates to a typewriter which is capable of being changed selectively between a print mode and a non-print mode during text creating operation.
2. Description of Prior Art
Known are electronic typewriters which are capable of being changed between a print mode and a non-print mode. One example is disclosed in U.S. patent application Ser. No. 479,810 assigned to the same assignee as hereof. In that typewriter, in the non-print mode, input data applied to the typewriter by operating the keyboard, is stored temporarily in a memory and displayed on a display unit, while the printer remains inactive. In the print mode, the input data is displayed on the display unit and is printed out by the printer. The typewriter is provided with a head pbsition register, which stores the position of the print head according to the printing operation; and an input position register which is adapted to be counted up on the basis of the input data applied to the typewriter, by operating the keyboard, for storing the position of a present input character in a current print line.
When this typewriter is operated in the print mode, the input position register or the head position counter is counted up as key operation or printing operation is executed. Initially, the input position register reads the contents of left margin position register and is counted up at each entry of data by the key operation. Upon coincidence of the contents of the input position register with the contents of right margin position register, any entry of data by the key operation is inhibited thereafter. Then, contents of the left margin position register are set again in the input position register by operation of the carriage return key. When the backspace key is operated, the input position register is counted down. When this electronic typewriter is used in the non-print mode, since the printing operation is not executed upon the key operation, the head position register and the input position register are not counted up.
Thus, in a conventional typewriter having no text memory, it has not been necessary to count up the input position register in the non-print mode, whereas in a typewriter having a text memory, the input position register needs to be counted up for formulating a text to be stored in the text memory while a text is being drawn up under the non-print mode. That is to say, the creation of text by use of left margin, right margin and tabulator is impossible. sible.
On the other hand, when the input position register is counted up in the non-print mode, since the contents of the left margin register are set in the input position register at each operation of the carriage return key, the contents of the input position register sometimes becomes smaller than the contents of the head position register. If the typewriter is changed from the non-print mode to the print mode in this state, the printing head is shifted toward the left margin by a distance corresponding to the difference betwecn the respective contents of the input position register and the head position register, and then executes printing operation. Accordingly, the printing head sometimes is shifted to the same position as the character or the like printed in the previous print mode, so that duplicate printing is liable to occur.